of Edinburgh, Session 1885 - 86 . 
895 
up a theory I had formed on the subject.* Mr Saville Kent and 
other observers f have come to the same opinion. This negative 
fact may therefore be considered as established. 
The sand and gravel that I found in the intestines of these 
animals were composed of dead coral, fragments of molluscan shells, 
small entire univalve and bivalve shells, foraminiferous tests, 
including the large OrUtolites , the joints of the calcareous alga, 
Halimeda ojpuntia, and other materials. Professor Semper, Professor 
Agassiz, and Mr Saville Kent have pointed out that the nutriment 
is chiefly obtained from the organic particles, diatoms, and foramini- 
fera, associated with the sand and gravel. Holothurians avoid 
these materials in enormous quantities. After several observations 
on one of the commonest species of the reef-flat, I arrived at the 
conclusion that each individual daily discharged not less than two- 
fifths of a pound (av.) of sand and gravel, and that in the course of 
a year fifteen or sixteen of these animals would discharge a ton of 
these materials, which would tend through this continual process of 
trituration and attrition to be reduced to the finest mud. Multitudes 
of other creatures take part in this operation of degradation, the 
first stage of which is to be seen in the destructive effect of the 
boring-molluscs, annelids, echinoids, sponges, &c., on the masses of 
dead corals. In the later stages, not only the holothurians, but all 
echinoderms (echinoids, asteroids, ophiuroids), together with fish, 
molluscs, and crustaceans are actively engaged. Thus by the 
process of organic degradation coral masses are reduced to fragments, 
and these again to sand and gravel, which in their turn are ground 
down into mud. Mr Darwin held that by this agency the fine 
sands and muds of the coral reefs are produced. His misconception 
respecting the habits of holothurians does not materially affect his 
conclusion. Of the importance of this agency I feel convinced. 
Professor A. Agassiz, however, contends that, as in the case of the 
Florida reefs, it is very slight as compared with the action of the 
breakers. These reefs, I would point out, are somewhat exceptionally 
situated. Here the rollers pass over the muddy slopes of a submarine 
bank before they reach the reefs, and in consequence, after storms, 
the sea is discoloured with chalky sediment for miles around. Such 
* Nature , Kov. 2, 1882, and Feb. 21, 1884. 
t Nature, March 8 and 29, 1883. 
